1. Field of the Invention
This invention relates to digital communication systems and more particularly to such systems where the interstation communication is via a continuous loop.
2. Description of the Prior Art
Digital communication systems of the continuous loop variety are now being looked to for use in situations where the various stations connected to the loop share a close physical proximity to each other. In some applications such systems are called closed-ring time division multiplex systems and may use pulse code modulation. Such a system is shown in U.S. Pat. No. 4,049,921 issued Sept. 20, 1977 to Edward Zwack, which patent is hereby incorporated by reference herein as if reproduced in full.
The Zwack patent discloses a closed loop communication system which allows conferencing between several stations. The stations are connected to the digital loop by an interface circuit which operates to add the instantaneous digital or analog signal onto the conference bus and to remove from the bus that portion of the combined signal which had been placed on the bus via the same interface circuit during the last cycle. The system operates with a delay circuit where the signal from the station is delayed one full cycle at the interface circuit and compared to the incoming signal for the purpose of subtracting from the incoming signal the portion of that combined signal which had been placed on the loop in the previous cycle. A problem exists with such systems in that, while the interface circuits operate to remove signals from the loop which were provided from the associated station nothing protects the loop from signals which have been erroneously placed thereon. The problem stems from the fact that the loop relies upon each interface circuit to remove from the loop those signals placed on the loop by that interface. However, those signals which, for one reason or another, enter the loop because of an error condition remain on the loop and, unless removed, tend to degrade communication.
Thus, it is one object of the invention to arrange a closed-loop communication system in a manner which serves to eliminate errors from the communication loop and to do so in a maner which does not increase the control time and in a manner which preserves the integrity of the loop.